1. Field of the Invention
The present invention relates to a method for measuring dimensions and an optical system using the same, and more particularly, to a method for measuring dimensions incorporating the through focus image-capturing technique and the focus metric algorithms and an optical system using the same.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
With the rapid growth and development of the semiconductor industry, the requirement to reduce the critical dimension (CD) and line-width has increased, and the precision of metrology tools must be raised to meet demands. According to Rayleigh criteria, it is quite difficult for conventional image metrology tools to meet the prospective resolution demands due to issues such as optical diffraction, precision of metrology tools and proximity effect. Consequently, it is necessary to find a new metrology to solve the above-mentioned problems. In addition, the metrology tools for measuring line-width and pitch, such as critical dimension scanning electron microscope (CD-SEM) and critical dimension atomic force microscope (CD-AFM) offer very high resolution, but are too expensive for most applications. Users need cost-effective and efficient image metrology tools, these can achieve the same resolution as CD-SEM and CD-AFM.
The conventional through focus method is proposed to increase the precision of metrology tools. The through focus method acquires multiple images and characteristic values from the image, and builds a relation of the characteristic value and the structural parameter such that an unknown structure parameter can then be determined based on the built relation. Consequently, the conventional image metrology tools can dramatically increase the precision without increasing the hardware cost, which has become an important approach for the image metrology tools in overcoming the optical diffraction limit issue. However, one critical factor to improve the precision of image metrology tools is developing a means of using this approach combined with a continuously improved algorithm to precisely determine unknown structural parameters.
FIG. 1 illustrates an optical system for measuring dimensions according to the prior art. The optical system comprises a lens 102 configured to acquire images of an object 100 with the lens 102 focusing on an imaging position 104, and an image sensor 106 positioned behind the imaging position 104 and configured to store the acquired images of the object 100 in a digital manner. The object 100 towards the lens 102 has an on-focus position, and the object 100 can be moved to an off-focus position from the on-focus position.
FIG. 2(a) and FIG. 2(b) are acquired images of the object 100 at an on-focus position and an off-focus position. The grating pattern of the object 100 is very clear if the object 100 is positioned at the on-focus position, as shown in FIG. 2(a). In contrast, the grating pattern of the object 100 is very unclear if the object 100 is moved to an off-focus position, as shown in FIG. 2(b). The conventional technique acquires a plurality of images at different positions, and the on-focus position is then determined based on the degree of clarity of the images. However, as the critical dimension shrinks down to below the resolution limit of the optical system, the grating pattern is not clear even if the object 100 is positioned right on the on-focus position, and the limitation of the conventional technology become apparent.